1. Field
The disclosed embodiments relate to a turbojet or turbojet engine for aircraft. More specifically, the disclosed embodiments relate to a thermal exchanger, also called a surface exchanger, housed in a turbojet. The thermal exchanger of the disclosed embodiments is designed for example to cool a fluid of the propulsion system of the turbojet, for example oil, so that it can be re-injected into said propulsion system in an at least partly cooled state. The disclosed embodiments also relate to an aircraft comprising at least one such turbojet.
In general, the thermal exchanger of the disclosed embodiments can be applied when a fluid designed to flow in a turbojet or on its periphery has to be cooled.
2. Description
There are known ways, in civil aviation, of using an ancillary thermal exchanger to cool the oil that circulates in the turbojet. The hot oil is conveyed to the thermal exchanger so as to be cooled therein before being re-utilized in the propulsion system.
FIG. 1 depicting the prior art provides a view in section of a turbojet 1 as well as two prior art thermal exchangers 6 and 12.
The turbojet 1 has a nacelle 2 housing an engine 3. The engine 3 is fixed to an inner wall 4 of the nacelle 4 by means of air bifurcations 5.
In the prior art, there are generally two possible ways of positioning the thermal exchanger: in the body of the engine 3 or in the nacelle 2.
When the thermal exchanger 6 is mounted in the body of the engine 3, it is more specifically housed in an internal volume 7 made between an engine hood 8 at least partially surrounding the engine 3, and the engine 3 itself. An air inlet 9 taps cold air from the cold air stream going through the turbojet 1, to convey it into the thermal exchanger 6. The cold air goes through the matrix of the thermal exchanger in which the hot oil to be cooled flows. The two fluids are separated from each other by partition walls and do not get mixed with each other. The calorific exchange takes place within the matrix. The partially heated air comes out of the thermal exchanger 6 through an air outlet 10 and is then re-injected into the secondary air stream coming out of the nacelle.
Should the thermal exchanger 12 be positioned at the nacelle 2, it is more specifically housed in the internal volume of said nacelle 2. An air inlet 13 taps cold air from the cold air stream going through the turbojet 1 to convey it into said thermal exchanger 12. After having crossed the matrix of the thermal exchanger 12, this airflow is either ejected out of the nacelle 2 by an air outlet 14 or re-introduced into the internal flow of the engine by a specific air outlet (not shown).
Thermal exchangers of this kind do not prove to be an optimum solution in terms of propulsion efficiency and aerodynamic impact on the engine. This is so for several reasons. When the air that goes through the matrix of the exchanger is expelled out of the internal flow of the engine, i.e. in the case of a mounting in the nacelle with an air outlet to the exterior, the tapping of air constitutes a direct loss of propulsion efficiency inasmuch as it makes no contribution or little contribution to the thrust of the engine. When the air that goes through the matrix of the thermal exchanger is re-introduced into the internal flow of the engine, in the case of a mounting in the body of the engine, the matrix of the thermal exchanger, by its internal architecture, induces a high loss of load in the flow and tends to create a disturbance of varying significance in the aerodynamic flow downstream from the engine. Besides, the presence of an air inlet and of one or more internal conduits as well as an air outlet gives rise to load losses and disturbs the internal flow of the engine to a variable extent.
Another known approach uses a plate-type exchanger. In particular, there is a known plate-type exchanger which locally matches the shape of the internal wall 4 of the nacelle 2 to which it is attached. An upper face of the thermal exchanger is attached to the inner wall 4 of the nacelle while a lower face is situated in the cold air stream that goes through the internal volume of the nacelle 2. The heat transported within the exchanger is transferred by thermal conduction to the internal face of the plate forming the lower face of said thermal exchanger. This hot plate is brushed against by the cold air stream flowing in the nacelle 2. The heat stored in the hot plate is thus dissipated by forced convection toward the aerodynamic flow of the turbojet 1.
One drawback of this second embodiment of a prior art thermal exchanger is that it is incompatible with the present-day systems for reducing sound nuisance coming out of the turbojet. Indeed, to reduce this sound nuisance, there are known ways of at least partially covering the internal wall for the nacelle 2 with an acoustic lining 11. More generally, this acoustic lining 11 covers the internal and external walls of the nacelle 2 and of the engine hood 8 when two of these walls are facing each other. The presence of this acoustic lining 11 is incompatible with the attachment of the plate-type thermal exchanger with the internal wall for the nacelle 2. In order to use such a plate-type thermal exchanger, it would be necessary to eliminate the acoustic lining 11 locally, and this proves to be difficult given the sizing criteria pertaining to sound nuisance.
The disclosed embodiments seek to provide a thermal exchanger capable of cooling a fluid such as oil or any other heat-conveying fluid playing a part in the propulsion system of the engine, that can be easily installed in a turbojet and adapts to current standards and constraints, especially acoustic standards and constraints. It is also sought to provide a thermal exchanger that has increased efficiency compared with the efficiency of the prior art thermal exchangers, i.e. has greater capacities of cooling.
To this end, the thermal exchanger of the disclosed embodiments are positioned in an internal volume of the nacelle of the turbojet without being attached either to the internal wall of the nacelle or to the external wall of the engine. Thus, the thermal exchanger of the disclosed embodiments have two thermal exchange surfaces, each of the surfaces being in contact with the flow of cold air going through the nacelle. The presence of these two thermal exchange surfaces increases the cooling capacities of said thermal exchanger. The thermal exchanger of the disclosed embodiments is positioned around the engine without being attached to it. The thermal exchanger of the disclosed embodiments are crossed by a hot fluid, such as hot oil. The heat thus transported within the thermal exchanger is transferred by thermal conduction to the internal surface of said thermal exchanger so as to be dissipated by forced convection towards the aerodynamic flow in which the thermal exchanger is immersed. The walls of the thermal exchanger are attached neither to the cold internal wall of the nacelle nor to the hot external wall of the turbojet. The thermal exchanger is optimized through the increase in surface area at which the thermal exchange can take place. Furthermore, no specific arrangement of the structures of the turbojet is necessary. The thermal exchanger is fitted to the nacelle without interfering with the existing structures.